Water-soluble, carboxyl-containing copolymers, the preparation thereof and the use thereof as scale inhibitor

ABSTRACT

Water-soluble copolymerizates are disclosed containing carboxyl groups, and also containing the following components incorporated by polymerization: (a) 30 to 80 mol % monoethylenically unsaturated C 3  -C 8  carboxylic acids, anhydrides thereof and/or water-soluble salts thereof; (b) 10 to 50 mol % C 2  -C 8  olefines, styrene, alkyl styrene, C 1  -C 10  alkyl vinyl ethers and/or vinyl esters of saturated C 1  -C 10  monocarboxylic acids; and (c) 0.01 to 20 mol % vinyl ethers and/or allyl ethers of alkoxylated C 1  -C 30  alcohols. The copolymerizates in question have a K-value of at least 8 (determined after H. Fikentscher). Also disclosed are methods of producing the water-soluble copolymerizates by copolymerization of the monomers in the presence of radical-forming polymerization initiators, and the use of the polymerizates as scale inhibitors.

The invention relates to water-soluble, carboxyl-containing copolymers,to a process for the preparation of the copolymers by polymerization ofmonoethylenically unsaturated carboxylic acids and other copolymerizablemonoethylenically unsaturated monomers in the presence of free-radicalpolymerization initiators and to the use of the copolymers as scaleinhibitor.

U.S. Pat. No. 3,756,257 discloses the use of homopolymers of acrylicacid as scale inhibitor. GB-A-1 411 063 discloses the use of hydrolyzedpolymaleic anhydrides in water treatment. Suitable polymers areprepared, for example, by polymerizing maleic anhydride in xylene usingbenzoyl peroxide as initiator and subsequently hydrolyzing thepolymaleic anhydride obtained in this way.

EP-B-0 009 169 discloses a process for the suspension copolymerizationof maleic anhydride and C₆ -C₁₀ -alkenes in excess alkenes as suspendingmedium. In this case, poly-C₁ -C₂₀ -alkyl vinyl ethers are used asdispersants.

EP-A-0 260 386 discloses the preparation of olefin-maleic anhydridecopolymers in powder form by free-radical polymerization of olefins andmaleic anhydride in aromatic solvents in the presence of copolymers ofethylene and vinyl acetate or polystyrene as dispersant.

GB-A-1 454 657 and EP-A-0 261 589 disclose the use of hydrolyzedcopolymers of maleic anhydride as scale inhibitor.

According to EP-A-0 276 464, water-soluble copolymers of maleic acid areprepared by copolymerization of maleic anhydride with a maximum of up to20% by weight of another monoethylenically unsaturated monomer which iscopolymerizable therewith in an aromatic hydrocarbon in the presence ofperoxy esters as polymerization initiators. The copolymerization can,according to the statements in the application, also be carried out inthe presence of protective colloids. After removal of the solvent andhydrolysis of the anhydride groups, the copolymers are used aswater-treatment agents for preventing deposition of scale and separationout of water hardness in water-conveying systems.

As disclosed in the abovementioned references, aggregation anddeposition of polymers on parts of the polymerization apparatus duringthe precipitation polymerization is avoided or substantially preventedon use of protective colloids. However, in this case, the copolymersresult in more or less coarse form and can often be dissolved completelyin water in the form of the salts only with difficulty. In most cases,cloudy salt solutions of the copolymers are obtained. However, when suchcloudy solutions are metered with the aid of metering pumps, blockagesin the metering pumps occur. Another problem in the use of copolymersof, for example, maleic anhydride and olefins is that aqueous solutionsof the copolymers foam greatly on use as scale inhibitor.

It is an object of the present invention to provide novel substanceswhich, for example when used as scale inhibitor, form clear solutionseven on dissolving in seawater and which virtually do not foam inseawater desalination by the multistage flash evaporation process.

We have found that this object is achieved by water-soluble,carboxyl-containing copolymers when they comprise

(a) 30 to 80 mol % of monoethylenically unsaturated C₃ -C₈ -carboxylicacids, their anhydrides and/or their water-soluble salts,

(b) 10 to 50 mol % of C₂ -C₈ -olefins, styrene, alkylstyrenes, C₁ -C₁₀-alkyl vinyl ethers and/or vinyl esters of saturated C₁ -C₁₀-monocarboxylic acids and

(c) 0.01 to 20 mol % of vinyl ethers and/or allyl ethers of alkoxylatedC₁ -C₃₀ -alcohols

as copolymerized units and have a K value of at least 8 (determined bythe method of H. Fikentscher on 1% by weight aqueous solutions of thesodium salts of the copolymers at 25° C. and pH 7.5).

The object is additionally achieved by a process for preparingwater-soluble copolymers by copolymerization of

(a) 30 to 80 mol % of monoethylenically unsaturated C₃ -C₈ -carboxylicacids, their anhydrides and/or water-soluble salts,

(b) 10 to 50 mol % of C₂ -C₈ -olefins, styrene, alkylstyrenes, C₁ -C₁₀-alkyl vinyl ethers and/or vinyl esters of saturated C₁ -C₁₀-monocarboxylic acids and

(c) 0.01 to 20 mol % of vinyl ethers and/or allyl ethers of alkoxylatedC₁ -C₃₀ -alcohols

in the presence of free-radical polymerization initiators.

The invention additionally relates to the use of the copolymersdescribed above as scale inhibitor.

The water-soluble copolymers comprise as monomers of group (a) from 30to 80, preferably 40 to 75, mol % of monoethylenically unsaturated C₃-C₈ -carboxylic acids, their anhydrides and/or water-soluble salts ascopolymerized units. Suitable carboxylic acids are, for example, acrylicacid, methacrylic acid, dimethylacrylic acid, ethacrylic acid,allylacetic acid, vinylacetic acid, crotonic acid, maleic acid, fumaricacid, citraconic acid, methylenemalonic acid, mesaconic acid anditaconic acid. Where said unsaturated carboxylic acids can formanhydrides, these anhydrides are also suitable as monomers of groups(a), eg. maleic anhydride, itaconic anhydride and methacrylic anhydride.The monoethylenically unsaturated carboxylic acids can also be used inthe copolymerization in the form of their water-soluble salts. Suitableexamples are the ammonium, alkali metal and alkaline earth metal saltsof these acids. They are obtained by partial or complete neutralizationof the unsaturated carboxylic acids with bases. Examples of suitablebases are ammonia, amines such as C₁ -C₃₀ -alkylamines, alkanolaminessuch as ethanolamine, diethanolamine or triethanolamine, morpholine, andsodium hydroxide solution, potassium hydroxide solution, sodiumcarbonate, potassium carbonate, sodium bicarbonate, potassiumbicarbonate, calcium hydroxide and barium hydroxide. Ammonia or sodiumhydroxide solution is preferably used to neutralize the carboxylicacids. The monomers of group (a) can be used in the copolymerizationeither alone or in a mixture. Apart from the partially neutralizedcarboxylic acids, suitable examples are mixtures of maleic anhydride andacrylic acid, mixtures of acrylic acid and itaconic anhydride, mixturesof maleic anhydride and itaconic anhydride and mixtures of maleicanhydride, acrylic acid and methacrylic acid. These mixtures can containthe constituents in any desired ratios. Maleic anhydride and mixtures ofmaleic anhydride and acrylic acid are preferred.

The copolymers comprise as monomers of group (b) from 10 to 50,preferably 15 to 49, mol % of C₂ -C₈ -olefins, styrene, alkylstyrenes,C₁ -C₁₀ -alkyl vinyl ethers and/or vinyl esters of saturated C₁ -C₁₀-monocarboxylic acids as copolymerized units. Examples of suitableolefins are ethylene, propylene, n-butene, isobutene, 1-pentene,1-hexene, 1-heptene and diisobutene. Apart from said α-olefins, thecorresponding isomers can also be used in the polymerization. Suitablealkylstyrenes contain 1 to 3 carbon atoms in the alkyl group, eg.α-methylstyrene. The alkyl vinyl ethers contain 1 to 10 carbon atoms inthe alkyl group, eg. vinyl methyl ether, vinyl ethyl ether, vinyln-propyl ether, vinyl isopropyl ether, vinyl n-butyl ether, vinylisobutyl ether, vinyl tert-butyl ether, vinyl hexyl ether and vinyl2-ethylhexyl ether.

Examples of vinyl esters of saturated C₁ -C₁₀ -monocarboxylic acids arevinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate and vinylpivalate. The monomers of group (b) can be used in the copolymerizationalone or in a mixture, eg. mixtures of styrene and isobutene or styreneand vinyl acetate or mixtures of isobutene and diisobutene. Thecopolymers preferably contain isobutene as copolymerized units.

The copolymers comprise as monomers of group (c) from 0.01 to 20,preferably 0.1 to 15, mol % of vinyl ethers and/or allyl ethers ofalkoxylated C₁ -C₃₀ -alcohols as copolymerized units. The alkoxylatedalcohols can be characterized, for example, by means of the followingformulae:

    R--O--(EO).sub.n --(PO).sub.m --(BuO).sub.o --H            (I),

where

R=C₁ -C₃₀ -alkyl,

EO=--CH₂ -CH₂ --O--, ##STR1## n, m, o=0 to 100, and the total of n+m+ois at least 3.

Examples of monomers of group (c) are the following compounds, where EO,PO and BuO have the meanings indicated in formula I:

CH₃ --O--(EO)₃ --CH═CH₂

CH₃ --O--(EO)₃ --CH₂ --CH═CH₂

CH₃ --O--(EO)₆ --CH═CH₂

CH₃ --O--(EO)₆ --CH₂ --CH═CH₂

CH₃ --O--(EO)₁₁ --CH═CH₂

CH₃ --O--(EO)₁₁ --CH₂ --CH═CH₂

CH₃ --O--(EO)₂₀ --CH═CH₂

CH₃ --O--(EO)₂₀ --CH₂ --CH═CH₂

CH₃ --O--(PO)₁₀ --CH═CH₂

CH₃ --O--(PO)₁₀ --CH₂ --CH═CH₂

CH₃ --O--(BuO)₁₀ --CH═CH₂

CH₃ --O--(BuO)₁₀ --CH₂ --CH═CH₂

CH₃ --O--(EO)₄ --(PO)₂ --CH═CH₂

CH₃ --O--(BuO)₂ --(EO)₇ --CH₂ --CH═CH₂

C₂ H₅ --O--(EO)₇ --CH═CH₂

C₂ H₅ --O--(EO)₁₀ --CH₂ --CH═CH₂

C₁₀ H₂₃ --O--(EO)₃ --CH₂ --CH═CH₂

C₁₀ H₂₃ --O--(EO)₅ --CH═CH₂

C₁₀ H₂₃ --O--(EO)₇ --CH═CH₂

C₁₀ H₂₃ --O--(EO)₁₁ --CH₂ --CH═CH₂

C₁₀ H₂₃ --O--BuO--PO--(EO)₂ --CH═CH₂

C₁₃ H₂₇ --O--(EO)₁₅ --CH═CH₂

C₁₃ H₂₇ --O--(EO)₂₀ --CH═CH₂

C₁₃ H₂₇ --O--(EO)₃₀ --CH₂ --CH═CH₂

C₁₆ H₃₃ --O--(EO)₅₀ --CH═CH₂

C₁₆ H₃₃ --O--(EO)₈₀ --CH₂ --CH═CH₂

C₁₈ H₃₇ --O--(EO)₅₀ --CH═CH₂

C₁₈ H₃₇ --O--(EO)₅₀ --CH₂ --CH═CH₂

C₄ H₉ --O--(PO)₂₄ --(EO)₄ --CH═CH₂.

The alkoxylated alcohols which underlie the monomers (c) and whichcontain at least two different alkylene oxides as copolymerized unitsmay contain the alkylene oxide units in the form of blocks or in randomdistribution. The monomers of group (c) which are preferably used arevinyl ethers of C₁ -C₁₈ -alkyl polyglycols.

The copolymers according to the invention are prepared bycopolymerization of the monomers of groups (a), (b) and (c) in thepresence of free-radical polymerization initiators. The copolymerizationcan moreover take place continuously or batchwise by all known processesof bulk, solution, precipitation, suspension and emulsionpolymerization. Suitable free-radical polymerization initiators are allperoxides, hydroperoxides, azo compounds and redox catalystsconventionally used for this purpose. Examples which may be mentionedare acetyl cyclohexanesulfonyl peroxide, diacetyl peroxydicarbonate,dicyclohexyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate,tert-butyl perneodecanoate,2,2-azobis(4-methoxy-2,4-dimethylvaleronitrile), tert-butyl perpivalate,tert-butyl per-2-ethylhexanoate, tert-butyl permaleate, tert-butylperisobutyrate, 2,2-azobis(isobutyronitrile),bis(tert-butylperoxy)cyclohexane, tert-butylperoxy isopropyl carbonate,tert-butyl peracetate, dicumyl peroxide, di-tert-amyl peroxide,di-tert-butyl peroxide, p-menthane hydroperoxide, cumene hydroperoxide,tert-butyl hydroperoxide, hydrogen peroxide, sodium peroxodisulfate,potassium peroxodisulfate, ammonium peroxodisulfate and peroxodisulfuricacid. It has in some cases proven useful to employ mixtures ofinitiators, in which case initiators which decompose at differenttemperatures are chosen. The polymerization initiators are used in theconventional amounts, ie. in amounts of from 0.1 to 20, preferably 0.2to 15, % of the weight of the monomers. The decomposition of theperoxides and the azo compounds can be speeded up by carrying out thecopolymerization in the presence of redox coinitiators such as benzoin,dimethylaniline, ascorbic acid and complexes, soluble in organicsolvents, of heavy metals such as copper, cobalt, manganese, iron,nickel and chromium. The amounts of coinitiators normally employed arefrom 0.1 to 2000, preferably 0.1 to 1000, ppm. The initiators orinitiator systems are preferably chosen so that their half-life at thepolymerization temperature is less than 3 hours.

The copolymerization can be carried out, for example, in the temperaturerange from 40 to 200, preferably 80 to 150, °C. If the reaction mixturecontains components whose boiling point is below the polymerizationtemperature, the copolymerization is carried out under pressure. Oxygenis excluded in all cases.

In order to prepare low molecular weight copolymers, it is oftenexpedient to carry out the copolymerization in the presence ofregulators. Examples of suitable regulators are allyl alcohol,1-buten-3-ol, organic mercapto compounds such as 2-mercaptoethanol,2-mercaptopropanol, mercaptobutanols, mercaptoacetic acid,mercaptopropionic acid, tert-butyl mercaptan, n-butyl mercaptan, n-octylmercaptan, n-dodecyl mercaptan and tert-dodecyl mercaptan. If aregulator or a mixture of several regulators is employed in thecopolymerization, the amounts used are from 0.1 to 10% of the weight ofthe monomers. The copolymerization can be carried out in conventionalapparatus, eg. in stirred vessels which are equipped, for example, withan anchor, paddle, impeller or multistage impulse countercurrentagitator. Continuous preparation of the copolymers takes place instirred vessel cascades, tubular reactors and static mixers. Thesimplest polymerization method is bulk polymerization. In this case, themonomers of groups (a) to (c) are polymerized in the presence of apolymerization initiator and in the absence of solvents and diluents. Itis expedient to mix all the monomers in the required composition,introduce a small part of the mixture, eg. 5-10%, into the reactor, heatthe mixture while stirring to the required polymerization temperature,and meter in the remaining monomer mixture and, separately therefrom,the initiator and, where appropriate, coinitiator and, whereappropriate, regulator over the course of from 1 to 10, preferably 2 to5, hours, continuously or at intervals. It may be expedient in this caseto meter in the initiator and possibly also the coinitiator separatelyfrom the monomers in the form of a solution in a small amount of asuitable solvent. The copolymers can be isolated immediately orprocessed to aqueous solutions ready for use. If they are not directlysoluble in water, they are partially or completely neutralized byaddition of a base.

Another process for preparing the copolymers is solution polymerization.It is carried out in solvents in which the monomers and the resultingcopolymers are soluble. Suitable for this purpose are all solvents whichmeet these conditions and do not react with the monomers under thepolymerization conditions. Examples of suitable solvents are acetone,methyl ethyl ketone, diethyl ketone, methyl isobutyl ketone, ethylacetate, butyl acetate, ethylene glycol dimethyl ether, diethyleneglycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycoldiethyl ether, tetrahydrofuran and dioxane. In order to prepare lowmolecular weight copolymers, tetrahydrofuran or dioxane is preferablyused as solvent because these solvents intervene as regulators in thepolymerization. As described above for bulk polymerization, it is alsoexpedient in this case to introduce the solvent and part of the monomermixture (eg. about 5-20%) into a polymerization reactor and to meter inthe remainder of the monomer mixture and the initiator and, whereappropriate, coinitiator and regulator. The concentration of themonomers in the solution polymerization can vary within wide limits, eg.from 20 to 80% by weight. The concentration of the monomers in thesolution employed for the polymerization is preferably from 30 to 70% byweight. The copolymer can easily be isolated from the solution aftercompletion of the polymerization by evaporation of the solvent. Thecopolymers are soluble in water at least in the form of their alkalimetal salts.

A preferred process for preparing the copolymers is precipitationpolymerization. The solvents employed in this case are those in whichthe monomers are soluble and the resulting copolymers are insoluble andtherefore precipitate. Examples of suitable solvents for precipitationpolymerization are ethers such as diethyl ether, dipropyl ether, dibutylether, methyl tert-butyl ether, diethylene glycol dimethyl ether,toluene, xylene, ethylbenzene, cumene, aliphatic hydrocarbons andmixtures of said solvents. As known from the prior art described at theoutset, it is expedient for precipitation polymerization processes totake place in the presence of a protective colloid in order to preventat least substantially the formation of aggregates of polymer particlesand deposition of the polymers on reactor wall and stirrer. Protectivecolloids which can be used are the substances described in theabovementioned references. They are very soluble in the solvents and donot react with the monomers. Suitable examples are copolymers of maleicanhydride with vinyl alkyl ethers and/or olefins with 4 to 20 carbonatoms and monoesters thereof with C₁₀ -C₂₀ -alcohols or mono- anddiamides with C₁₀ -C₂₀ -alkylamines, and polyalkyl vinyl ethers whosealkyl group contains from 1 to 20 carbon atoms, such as polymethyl vinylether, polyethyl vinyl ether, polyisobutyl vinyl ether and polyoctadecylvinyl ether. The protective colloids normally have K values of from 20to 100 (determined by the method of H. Fikentscher in 1% by weightsolution in cyclohexanone at 25° C.).

Protective colloids are normally used in the precipitationpolymerization when the concentration of monomers in the solvent is morethan 30% by weight. The amounts of protective colloid added are normallyfrom 0.05 to 4% of the weight of the monomers. It may be advantageous insome cases to combine several protective colloids. For example, it isexpedient in precipitation polymerization to introduce the solvent,protective colloid and part of the monomer mixture into the reactor andto meter in at the chosen polymerization temperature, while stirringvigorously, the remainder of the monomer mixture and initiator and,where appropriate, coinitiator and regulator. The feed times for themonomer mixture and initiator are, in general, from 1 to 10, preferably2 to 5, hours. However, it is also possible to polymerize all thestarting materials together in one reactor, but in this case problemsmay arise with the dissipation of heat. The concentrations of themonomers to be polymerized in the solvent are, for example, from 10 to80, preferably 30 to 70, % by weight. After completion of thepolymerization, the polymers can be immediately isolated and freed ofadherent solvent in evaporators, belt driers, paddle driers, spraydriers or fluidized bed driers. It is then possible to prepare aqueoussolutions from the copolymer powders or aqueous solutions of the saltsof the copolymers by neutralization. A preferred method for preparingthe aqueous salt solution of the copolymers comprises initially addingwater to the polymer suspension resulting from the polymerization andthen removing the solvent by steam distillation. The neutralization thentakes place by adding a base. Bases suitable for the neutralization havealready been mentioned above. Suitable and preferred for the partial orcomplete neutralization of the copolymers are sodium hydroxide,potassium hydroxide, magnesium hydroxide, ammonia, ethanolamine,diethanolamine and triethanolamine. The copolymers are soluble in waterat least in the salt form. The aqueous solutions are brown to colorlessand clear.

The copolymers have K values of at least 8 (determined by the method ofH. Fikentscher on 1% by weight aqueous solutions of the sodium salts ofthe copolymers at 25° C. and pH 7.5). The K values of the copolymers canbe up to about 300. Preferred water-soluble copolymers are those whichcomprise

(a) 40-75 mol % of maleic acid, maleic anhydride, acrylic acid and/ormethacrylic acid,

(b) 15-49 mol % of isobutene, diisobutene, methyl vinyl ether, styreneand/or vinyl acetate and

(c) 0.1-15 mol % of vinyl ethers of C₁ -C₁₈ -alcohols onto which from 3to 20 mol of ethylene oxide and/or propylene oxide have been added,

as copolymerized units and have K values of from 10 to 100.

The copolymers can be used in various sectors, eg. as dispersants forproducing plastic, wax or oil dispersions or as dispersants for pigmentsfor producing highly concentrated aqueous pigment suspensions. Thecopolymers can be used, depending on their composition, as dispersantsfor oil-in-water dispersions or as dispersants for water-in-oildispersions. The copolymers are preferably used as scale inhibitors.They prevent the deposition of scale and the separation out of waterhardness in water-conveying systems. The mode of action of thecopolymers according to the invention as scale inhibitors comprises, inparticular, the prevention of the formation of firmly adherentprecipitates of the hardness salts such as calcium carbonate, magnesiumoxide, magnesium carbonate, calcium, barium or strontium sulfate,calcium phosphate and similar alkaline earth metal salts of lowsolubility. Addition of the copolymers in the substoichiometric dosagerange influences the formation of deposits in such a way that no hardand rock-like deposits are produced, on the contrary there is formationof deposits which are finely divided in water and can easily besuspended. In this way, the surfaces of, for example, heat exchangers,pipes or pump components are kept free of deposits, and their tendencyto corrosion is greatly reduced. In particular, the risk of crevicecorrosion is reduced thereby. Furthermore, the growth of microorganismson these metal surfaces is impeded. The scale inhibitors are able toincrease the useful lives of such systems and considerably reducestoppage times for cleaning systems components. Water-conveying systemsto which a scale inhibitor is added comprise, for example, open orclosed cooling circulations, for example of power stations or chemicalfacilities such as reactors, stills or similar components where heatmust be dissipated. Scale inhibitors can also be employed in boilerwaters and steam generators. A preferred application of the scaleinhibitors to be used according to the invention is the desalination ofseawater by distillation or by means of membrane processes such asreverse osmosis or electrodialysis. Thus, for example, in the multistageflash evaporation distillation process for seawater desalination,concentrated seawater is circulated at elevated temperature. In thiscase, the scale inhibitors effectively suppress the separation out ofhardness components such as brucite and the caking thereof ontocomponents of the system. The amounts of copolymer required for scaleinhibition are, for example, from 0.1 to 100 ppm copolymer based on theaqueous medium. Copolymers preferably used as scale inhibitors have a Kvalue of from 10 to 50 (determined by the method of H. Fikentscher on 1%by weight aqueous solutions of the sodium salts at 25° C. and pH 7.5).

The copolymers can also be used as scale inhibitors in the evaporationof juices from cane or beet sugar. As is known, calcium hydroxide,carbon dioxide, sulfur dioxide and, where appropriate, phosphoric acidare added to the sugar thin juice for purification. Calcium salts of lowsolubility, such as calcium carbonate, calcium sulfate or calciumphosphate, remain after filtration of the sugar juice treated in thisway. These salts then precipitate during the evaporation process and mayform rock-like deposits on heat exchanger surfaces. Rock-like depositsof concomitant substances such as silica or calcium salts of organicacids such as oxalic acid are also produced. The copolymers to be usedaccording to the invention also act as scale inhibitors on addition tosugar thin juice and prevent the formation of rock-like deposits in theequipment where the sugar thin juice is evaporated.

The copolymers can also be used as incrustation inhibitors andantiredeposition agents in amounts of from 0.5 to 10% by weight indetergent formulations. The copolymers are normally used in the form ofdilute aqueous solutions. The particular advantage of the copolymersaccording to the invention compared with known copolymers is that noturbidity occurs on dilution of the aqueous copolymer solutions withseawater, so that such solutions can be metered without difficulties.Compared with known copolymers, the copolymers according to theinvention have the additional advantage that there is a great reductionin the foam formation on evaporation of seawater.

The K values of the copolymers were determined by the method of H.Fikentscher, Cellulose-Chemie, 13 (1932) 48-64 and 71-74 in 1% aqueoussolution of the sodium salts of the copolymers at 25° C. and pH 7.5. Thepercentage data mean percent by weight.

EXAMPLES 1 TO 10

Preparation of copolymers

2700 g of the solvent indicated in each case in Table 1, and 6 g of2-mercaptoethanol in each of Examples 1 and 2 are introduced into aheatable pressure reactor equipped with a stirrer and feed devices andare heated to the temperature indicated in each case in Table 1. As soonas this temperature is reached, there is addition at a constant rate ofmonomers (a), (b) and (c) indicated in each case in Table 1, in 300 g ofthe solvent which is likewise indicated in Table 1, over the course of 4hours and, separately therefrom, of a solution of 6 g of tert-butylperpivalate in 100 g of the solvent used in each case over the course of5 hours. After addition of the initiator is complete, the reactionmixture is stirred at the temperature stated in the Table for 2 hoursand then cooled, and 300 g of water are added. The solvent is removed bypassing steam in, and the pH is adjusted to 7.5 by adding 50% strengthaqueous sodium hydroxide solution. The 40% strength aqueous polymersolutions obtainable in this way are almost colorless (Example 6),yellowish (Examples 4, 5 and 9) or brownish (Examples 1, 2, 3, 7, 8 and10). The aqueous solutions are in all cases clear.

                                      TABLE 1                                     __________________________________________________________________________    Monomers  g!                                   Temp.                          Ex.                                                                              a)     b        c)                   Solvent                                                                               °C.!                                                                      K value                    __________________________________________________________________________    1  210.9 g                                                                           MA 55.6 g                                                                            Isobutene                                                                          33.4 g                                                                            CH.sub.3 (C.sub.2 H.sub.4 O).sub.3 --O--CH═CH.s                           ub.2             Toluene                                                                              100 19.8                       2  209.8 g                                                                           MA 55.3 g                                                                            Isobutene                                                                          34.9 g                                                                            CH.sub.3 --(C.sub.2 H.sub.4 O).sub.3 --O--CH.sub.2                            --CH═CH.sub.2                                                                              Toluene                                                                              100 17.4                       3  183.8 g                                                                           MA 30 g                                                                              Isobutene                                                                          86.2 g                                                                            CH.sub.3 --(C.sub.2 H.sub.4 O).sub.6 --O--CH═CH                           .sub.2           o-Xylene                                                                             120 22.5                       4  177.1 g                                                                           MA 58.7 g                                                                            Isobutene                                                                          122 g                                                                             CH.sub.3 --(C.sub.2 H.sub.4 O).sub.6 --O--CH.sub.2                            --CH═CH.sub.2                                                                              o-Xylene                                                                             120 26.5                       5  164.5 g                                                                           MA 42.8 g                                                                            Isobutene                                                                          92.7 g                                                                            CH.sub.3 --(C.sub.2 H.sub.4 O).sub.15 --O--CH═C                           H.sub.2          Ethylbenzene                                                                         120 18.3                       6  225 g                                                                             MA 53.3 g                                                                            Isobutene                                                                          21.7 g                                                                            C.sub.10 H.sub.21 --(C.sub.2 H.sub.4 O).sub.3                                 --O--CH═CH.sub.2                                                                           Toluene                                                                               90 43.5                       7  141.6 g                                                                           MA 111.8 g                                                                           Vinyl                                                                              46.6 g                                                                            CH.sub.3 --(C.sub.2 H.sub.4 O).sub.6 --O--CH═CH                           .sub.2           o-Xylene                                                                             120 18.7                                     acetate                                                         8  185.9 g                                                                           MA 70.5 g                                                                            Styrene                                                                            43.6 g                                                                            CH.sub.3 --(C.sub.2 H.sub.4 O).sub.6 --O--CH═CH                           .sub.2           o-Xylene                                                                             120 20.5                       9  161.2 g                                                                           MA 85.8 g                                                                            Methyl                                                                             53 g                                                                              CH.sub.3 --(C.sub.2 H.sub.4 O).sub.6 --O--CH═CH                           .sub.2           Toluene                                                                               90 32.5                                     vinyl ether                                                     10 152 g                                                                             MA 53 g                                                                              Isobutene                                                                          50 g                                                                              CH.sub.3 --(C.sub.2 H.sub.4 O).sub.6 --O--CH═CH                           .sub.2           o-Xylene                                                                             120 19.6                          45 g                                                                              AA                                                                     __________________________________________________________________________     MA: Maleic anhydride                                                          AA: Acrylic acid                                                         

In order to determine the use properties of the copolymers, theirbehavior in seawater desalination, on dilution with 2% strength aqueoussodium chloride solution and in foam formation on passing air through isassessed. The following test methods were used for this:

Scale inhibition in seawater desalination

In this test, the copolymers are investigated as inhibitor in theformation of calcium and magnesium hydroxides and carbonates fromsynthetic seawater solution. During seawater desalination, in particularcalcium carbonate and magnesium hydroxide form firmly adherent andinterfering deposits on the heat exchanger surfaces. The formation ofbarium sulfate is also a problem which must be taken seriously in thisconnection. The test solution comprises an aqueous salt solution whichcontains

70° German hardness of Mg²⁺

14° German hardness of Ca²⁺

70° German hardness of CO₃ ²⁻.

The synthetic seawater solution is then mixed in each case with 25 ppmof the copolymers described in Examples 1 to 10 and pumped for 3 hoursthrough a dynamic circulation apparatus. After the test has lasted 3hours, samples are taken and analyzed by titrimetry for the content ofwater hardness. The deposits forming in the heat exchanger can becalculated from the decrease in water hardness during the test. Asmaller degree of hardness in the removed water sample means moredeposits on the heat exchanger pipes. The water hardnesses found arelisted in Table 2. A high degree of hardness after 3 hours of the testindicates good scale inhibition.

Salt stability in solution

In order to test the salt stability of the 40% strength aqueouscopolymer solutions obtained as in Examples 1 to 10, the solutions areeach diluted to a copolymer content of 10% by adding 2% strength sodiumchloride solution. After storage in a heat cabinet for 24 hours, theappearance of the diluted solution is assessed. The results are shown inTable 2.

Determination of foam formation

The 40% strength aqueous copolymer solutions indicated in Table 1 areeach diluted to a copolymer content of 10% by weight with distilledwater. 200 ml of the diluted solution are then introduced into a foamtube with a capacity of 1500 ml. 13.5 1 of air per hour are passed intothe foam tube from below through a sintered disk. 5 minutes afterstarting to pass air in, the height of the foam above the liquid levelis measured. The results of measurement are indicated in Table 2 and arecompared therein with the results of measurement for ComparativeExamples 1 to 5.

COMPARATIVE EXAMPLE 1

A hydrolyzed homopolymer of maleic acid was prepared as disclosed inGB-A-1 411 063, Example 1, by polymerizing maleic anhydride in xylenewith di-tert-butyl peroxide as catalyst at 130° C.

COMPARATIVE EXAMPLE 2

A hydrolyzed homopolymer of maleic anhydride was prepared as disclosedin EP-A-0 261 589, Example 6, by polymerizing maleic anhydride in xylenewith tert-butyl per-2-ethylhexanoate at the boiling point of the xylene.

COMPARATIVE EXAMPLE 3

A terpolymer of maleic anhydride, vinyl acetate and ethyl acrylate inthe molar ratio 9:2:1 was tested as scale inhibitor as disclosed inExample 14 of GB-A-1 454 657.

COMPARATIVE EXAMPLE 4

The copolymer of maleic anhydride and hydroxypropyl acrylate describedin Example 1 of EP-A-0 276 464 was tested as scale inhibitor.

COMPARATIVE EXAMPLE 5

The test solution described above was investigated without any otheradditive.

                  TABLE 2                                                         ______________________________________                                                Scale inhibition -  Foam height av.                                           water hardness                                                                          Salt solution                                                                           of 3 measurements                                 Ex.  Comp. Ex.                                                                              Start   after 3 h                                                                           after 24 h                                                                            in mm                                     ______________________________________                                        11   --       84      65    clear   10                                        12   --       84      69    clear   10                                        13   --       84      63    clear    0                                        14   --       84      68    clear   10                                        15   --       84      62    clear    0                                        16   --       84      66    clear   15                                        17   --       84      60    clear   35                                        18   --       84      63    clear   40                                        19   --       84      59    clear   10                                        20   --       84      70    clear    5                                             1        84      49    insoluble                                                                             55                                                                    portions                                               2        84      50    insoluble                                                                             50                                                                    portions                                               3        84      55    clear   45                                             4        84      52    clear   30                                             5        84      42    --      --                                        ______________________________________                                    

We claim:
 1. A method of scale inhibition comprising:adding to a systemin need thereof, a copolymer which is obtained by copolymerizing (a) 30to 80 mol % of monoethylenically unsaturated C₃ -C₈ -carboxylic acids,their anhydrides and/or their water-soluble salts, (b) 10 to 50 mol % ofC₂ -C₈ -olefins, styrene, alkylstyrenes, C₁ -C₁₀ -alkyl vinyl ethersand/or vinyl esters of saturated C₁ -C₁₀ -monocarboxylic acids and (c)0.01 to 20 mol % of vinyl ethers of alkoxylated C₁ -C₃₀ -alcohols in thepresence of free radical polymerization initiators.
 2. The method ofclaim 1, wherein said vinyl ethers of alkoxylated C₁ -C₃₀ -alcohols isof the formula

    R--O--AO--CH═CH.sub.2,

wherein R=C₁₋₃₀ alkyl and AO are alkoxylation units.
 3. The method ofclaim 1, wherein said vinyl ethers of alkoxylated C₁ -C₃₀ -alcohol is ofthe formula

    R--O--(EO).sub.n --(PO).sub.m --(BuO).sub.o --CH═CH.sub.2

wherein R=C₁₋₃₀ alkyl, EO=CH₂ -CH₂ --O--; ##STR2## n, m, o=0 to 100, andthe total of n+m+o is at least
 3. 4. The method of claim 1, wherein saidvinyl ether of alkoxylated C₁ -C₃₀ -alcohol is selected from the groupconsisting of the formulaCH₃ --O--(EO)₃ --CH═CH₂, CH₃ --O--(EO)₆--CH═CH₂, CH₃ --O--(EO)₁₁ --CH═CH₂, CH₃ --O--(EO)₂₀ --CH═CH₂, CH₃--O--(PO)₁₀ --CH═CH₂, CH₃ --O--(BuO)₁₀ --CH═CH₂, CH₃ --O--(EO)₄ --(PO)₂--CH═CH₂, C₂ H₅ --O--(EO)₇ --CH═CH₂, C₁₀ H₂₃ --O--(EO)₅ --CH═CH₂, C₁₀H₂₃ --O--(EO)₇ --CH═CH₂, C₁₀ H₂₃ --O--BuO--PO--(EO)₂ --CH═CH₂, C₁₃ H₂₇--O--(EO)₁₅ --CH═CH₂, C₁₃ H₂₇ --O--(EO)₂₀ --CH═CH₂, C₁₆ H₃₃ --O--(EO)₅₀--CH═CH₂, C₁ ₈ H₃₇ --O--(EO)₅₀ --CH═CH₂, C₄ H₉ --O--(PO)₂₄ --(EO)₄--CH═CH₂.and a mixture thereof.